1. Field of the Invention
The present invention relates to a printed circuit board and a method for producing the same.
2. Description of Related Art
Accompanying the reduction in size, thickness and weight of electronic equipment and the improvement in performance thereof in recent years, various technologies enabling a high-density packaging have been developed actively not only in various types of electronic components constituting the electronic equipment, but also in printed circuit boards on which these electronic components are mounted. In particular, along with the rapid advancement of the packaging technology recently, there is an increasing demand for low-cost multilayered circuit boards that can both achieve a high-density packaging of semiconductor chips such as LSIs and respond to high speed circuitry. In such printed circuit boards, it is important to have high electrical connection reliability between wiring patterns, which are formed on multiple layers with a fine wiring pitch, and high frequency characteristics.
In response to this, a printed circuit board in which layers are electrically connected using an electrically conductive paste has been proposed recently in JP 2601128 B. FIGS. 6A to 6G show a method for producing this printed circuit board. First, as shown in FIG. 6A, release films 501 made of polyester or the like are laminated on both surfaces of a porous base material 502 such as an aramid-epoxy prepreg that is obtained by impregnating an aramid nonwoven fabric with a thermosetting epoxy resin. Next, as shown in FIG. 6B, through holes 503 are formed at predetermined positions in the porous base material 502 by laser processing. Then, the through holes 503 are filled with an electrically conductive paste 504 as shown in FIG. 6C. As a filling method, the porous base material 502 with the through holes 503 is placed on a table of a screen printing machine and the electrically conductive paste 504 is printed directly over one of the release films 501. In this case, the release film 501 on the printed side serves as a print mask and to prevent the surface of the porous base material 502 from being contaminated. Subsequently, the release films 501 are peeled off from the both surfaces of the porous base material 502. Then, metal foils 505 such as copper foils are laminated on the both surfaces of the porous base material 502. With this state maintained, heating and compression are carried out, so that the porous base material 502 is compressed to be thinner as shown in FIG. 6D. Simultaneously, the electrically conductive paste 504 within the through holes 503 also is compressed, and a binder component contained in the electrically conductive paste 504 is forced out, thus strengthening the adhesion between electrically conductive components and between the electrically conductive component and the metal foils 505. As a result, the electrically conductive substance contained in the electrically conductive paste 504 becomes dense, thus achieving an electrical connection between layers. Thereafter, the thermosetting resin constituents of the porous base material 502 and the electrically conductive paste 504 are cured. Then, as shown in FIG. 6E, the metal foils 505 are etched selectively into a predetermined pattern, thus completing a double-sided circuit board. Furthermore, as shown in FIG. 6F, porous base materials 506 on which an electrically conductive paste 508 is printed and metal foils 507 are attached on both sides of the double-sided circuit board, followed by heating and compression. Subsequently, as shown in FIG. 6G, the metal foils 507 are etched selectively into a predetermined pattern, thus completing a multilayered circuit board.
However, in the structure and the producing method described above, when the aramid-epoxy prepreg is used, there is a slight deterioration in characteristics observed under a severe environment where electronic equipment causing a sharp temperature change is used. Accordingly, resin circuit boards with still higher reliability have been desired.
In order to solve these problems, it has been considered that a glass-epoxy prepreg obtained by impregnating a glass cloth with a thermosetting epoxy resin is used as an electrical insulating base material. However, in the glass-epoxy prepreg, resin layers having the same thickness are formed on both sides of the glass cloth. Accordingly, when the copper foil is laminated on one surface and the substrate on which a certain pattern is formed is laminated on the other surface, an excessively large amount of the resin causes a resin flow on the copper foil side, making it difficult to obtain connection reliability, while an excessively small amount thereof makes it difficult to obtain a sufficient adhesion on the patterned layer side. Alternatively, when the patterned substrates having a different thickness are laminated on the respective surfaces, an excessively large amount of the resin causes a resin flow on the side of the thinner patterned layer, making it difficult to obtain connection reliability, while an excessively small amount thereof makes it difficult to obtain a sufficient adhesion on the side of the thicker patterned layer.
It is an object of the present invention to solve the conventional problems described above and to provide a printed circuit board and a method for producing the same, in which, by adjusting the thickness of resin layers formed on both sides of a resin holder such as a glass cloth, high reliability can be achieved when layers are electrically connected by an electrical conductor such as an electrically conductive paste.
In order to achieve the above-mentioned object, a printed circuit board of the present invention includes an electrical insulating base material with through holes that are formed in a thickness direction of the electrical insulating base material and are filled with an electrical conductor; the electrical insulating base material including a core layer formed by impregnating a holder with a resin and resin layers formed on both sides of the core layer; and wiring layers that are formed on both surfaces of the electrical insulating base material into a predetermined pattern and are electrically connected to each other by the electrical conductor. The wiring layer is embedded in at least one of the resin layers. The resin layers on the both sides have different thicknesses from each other, and a thinner layer out of the resin layers has a thickness equal to or smaller than a mean particle diameter of an electrically conductive filler contained in the electrical conductor.
Furthermore, a method for producing a printed circuit board of the present invention includes laminating release films on both surfaces of an electrical insulating base material, the electrical insulating base material including a core layer including a prepreg formed by impregnating a holder with a resin and resin layers that are formed on both sides of the core layer and have different thicknesses from each other, providing through holes in a thickness direction of the electrical insulating base material, filling the through holes with an electrically conductive paste, superposing a wiring base material, on which a wiring layer is formed into a predetermined pattern so as to correspond to a portion filled with the electrically conductive paste, on at least one surface of the electrical insulating base material, and embedding the wiring layer in at least one of the resin layers on the electrical insulating base material by heating and compressing the electrical insulating base material on which the wiring base material has been superposed.
The present invention can form a via hole and a wiring layer with high reliability. In other words, at least one of the wiring layers is embedded in the resin layer, thereby compressing the electrical conductor within the through holes sufficiently. As a result, a conductor component of the electrical conductor becomes dense, thus allowing a via-hole connection with high reliability. In addition, the resin layers on both sides have a different thickness. Therefore, when the copper foil and the substrate on which different patterns are formed are laminated, or when the patterned substrates having a different thickness are laminated, it is possible to design the thickness of those resin layers so as to achieve high connection reliability and adhesive strength.